Start using the dart2js type conversions in Dartium as well.

sdk/lib/html/html_common/conversions_dartium.dart

+part of html_common;
+
+convertDartToNative_PrepareForStructuredClone(value) =>
+    new _StructuredCloneDartium().convertDartToNative_PrepareForStructuredClone(value);
+
+convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
+    new _AcceptStructuredCloneDartium().convertNativeToDart_AcceptStructuredClone(object, mustCopy: mustCopy);
+
+class _StructuredCloneDartium extends _StructuredClone {
+  newJsMap() => new js.JsObject(js.context["Object"]);
+  putIntoMap(map, key, value) => map[key] = value;
+  newJSList(length) => new js.JsArray();
+  cloneNotRequired(e) => e is js.JsObject;
+}
+
+class _AcceptStructuredCloneDartium extends _AcceptStructuredClone {
+  newDartList(length) => new List(length);
+
+  // JsObjects won't be identical, but will be equal only if the underlying
+  // Js entities are identical.
+  bool identicalInJs(a, b) =>
+      (a is js.JsObject) ? a == b : identical(a, b);
+
+  void forEachJsField(jsObject, action) {
+    var keys = js.context["Object"].callMethod("keys", [jsObject]);
+    for (var key in keys) {
+      action(key, jsObject[key]);
+    }
+  }
+}
+
+final _dateConstructor = js.context["Date"];
+final _regexConstructor = js.context["RegExp"];
+
+bool isJavaScriptDate(value) => value is js.JsObject && value.instanceof(_dateConstructor);
+bool isJavaScriptRegExp(value) => value is js.JsObject && value.instanceof(_regexConstructor);
+bool isJavaScriptArray(value) => value is js.JsArray;
+
+final _object = js.context["Object"];
+final _getPrototypeOf = _object["getPrototypeOf"];
+_getProto(object) {
+  return _getPrototypeOf.apply([object]);
+}
+final _objectProto = js.context["Object"]["prototype"];
+
+bool isJavaScriptSimpleObject(value) {
+  if (!value is js.JsObject) return false;
+  var proto = _getProto(value);
+  return proto == _objectProto || proto == null;
+}
+bool isImmutableJavaScriptArray(value) =>
+    isJavaScriptArray(value) && value["immutable$list"] != null;
+
+final _promiseConstructor = js.context['Promise'];
+bool isJavaScriptPromise(value) => value is js.JsObject && value['constructor'] == _promiseConstructor;
+
+Future convertNativePromiseToDartFuture(js.JsObject promise) {
+  var completer = new Completer();
+  var newPromise = promise
+    .callMethod("then", [(result) => completer.complete(result)])
+    .callMethod("catch", [(result) => completer.completeError(result)]);
+  return completer.future;
+}